Work machines such as, for example, excavators, loaders, dozers, motor graders, and other types of heavy machinery use multiple hydraulic actuators to accomplish a variety of tasks. These actuators are typically velocity controlled based on an actuation position of an operator interface device. For example, an operator interface device such as a joystick, a pedal, or any other suitable operator interface device may be movable to generate a signal indicative of a desired velocity of an associated hydraulic actuator. When an operator moves the interface device, the operator expects the hydraulic actuator to move at an associated predetermined velocity. However, this predetermined velocity is set during manufacture of the work machine, generally without a load being applied to the hydraulic actuator. During operation of the work machine when a load applied against the hydraulic actuator is light, the hydraulic actuator may move at a velocity that substantially matches the operator's expected velocity. However, when the load applied against the hydraulic actuator is heavy, the hydraulic actuator may move at slower and unexpected or undesired velocity. Also, when the load changes direction, the hydraulic actuator may move faster than expected, resulting in voiding within the hydraulic actuator. Attempts to control the velocity of the hydraulic actuator regardless of loading have resulted in harsh or jerky movements of the hydraulic actuator.
One method of improving the predictability of hydraulic actuator velocity while providing smooth operation of they hydraulic actuator is described in U.S. Pat. No. 6,880,332 (the '332 patent) issued to Pfaff et al. on Apr. 19, 2005. The '332 patent describes a hydraulic actuator controlled by electrohydraulic proportional valves to operate in different metering modes. A joystick position signal is converted into a desired velocity signal for the hydraulic actuator. The desired velocity signal is then used to command an opening amount of each of the electrohydraulic proportional valves to drive the hydraulic actuator at the desired velocity. A load on the hydraulic actuator is determined by measuring pressures associated with the hydraulic actuator, and the hydraulic actuator is operated in the different modes based on the determined load. A transition strategy is used to transition between the modes of operation, wherein the supply and return line pressures are set to threshold pressures required for the new mode of operation before transitioning from the old mode of operation.
Although the hydraulic actuator and control strategy of the '332 patent may improve velocity predictability of the fluid actuator under varying loads by basing the mode of operation on measured loading conditions, it may be complicated and still lack sufficient control. In particular, because each of the electrohydraulic proportional valves are controlled based on the desired velocity signal, the control strategy may be complex and require precise timing and calibration to avoid undesired valve interactions. In addition, because the opening amount of the valves is based solely on desired velocity, pressure fluctuations can still adversely affect predictability of the hydraulic actuator.
The disclosed hydraulic control system is directed to overcoming one or more of the problems set forth above.